The present invention relates to a charge and discharge control circuit for controlling the charge and discharge of a secondary cell, and to a chargeable power source device using said circuit.
A chargeable power source device has been disclosed in the prior art as shown in a circuit block diagram of FIG. 2. The construction such as FIG. 2, for example, is shown in Provisional Publication No. H4-74530 "A chargeable power source device". A secondary cell 201 is connected to an external terminal 205 or 204 through a switch circuit 203. Further, a charge and discharge control circuit 202 is connected to the secondary cell 201 in parallel. The charge and discharge control circuit 202 has a function to detect voltage of the secondary cell 201. A signal is generated by the charge and discharge control circuit 202 so as to turn the switch circuit 203 OFF at either of overcharge state (the voltage of the cell is higher than a fixed voltage) and an overdischarge state (the voltage of the cell is lower than a fixed voltage) of the secondary cell 201. By stopping discharge when the external terminal 204 reaches a certain voltage, it is also possible to limit current flowing through the switch circuit 203. This means that discharge can be stopped (overcurrent control) when overcurrent flows. This operation is hereinafter referred to as overcurrent protection.
A numeral 210 shows a positive pole of the secondary cell 201 and a numeral 211 shows a negative pole of the secondary cell 201.
Numerals 206 and 207 each show a signal line,respectively.
A numeral 208 shows a charger and a numeral 209 shows a load.
As another example of the typical charge and discharge power source device comprising a secondary cell, a power supply device such as shown in a circuit block diagram of FIG. 3 is known. In the circuit, the switch circuit 303 is connected in series to a negative pole 311 of the secondary cell 301.
However, the charge and discharge control circuit constructed like that above does not detect when the voltage of the cell is less than a threshold voltage of a MOS transistor, and a problem occurs when a charger is connected.
A numeral 310 shows a positive pole of the secondary cell 301.
A numeral 302 shows a charge and discharge control circuit.
Numerals 304 and 305 each show an external terminal, respectively.
Numerals 306 and 307 each show a signal line,respectively.
A numeral 309 shows a load.
In general, two FETs (Field Effect Transistor) are used for the switch circuit. As an another example of a device using the same switch circuit, a power source device shown in circuit block diagram in FIG. 4 in which, a switch circuit 403 comprises two FETs.
The switch circuit operates so as to turn an FET 412 OFF in an overdischarge state and to turn an FET 413 OFF in an overcharge state. Because of that, a signal line for switch circuit control is divided two, 407A and 407B. Also in overcurrent state, the circuit operates so as to turn the FET 412 OFF.
In this circuit, when a charger 408 is connected at the over-discharge state of cell voltage as shown in FIG. 4, charge current flows through a parasitic diode of the FET so as to charge a secondary cell 401 because the FET 412 is OFF and the FET 413 is ON. This occurs during ordinary circuit operation in the normal cell condition. However, even when the voltage of the cell is less than a threshold voltage of a MOS transistor, the circuit operates so as to let charge current flow and to charge the cell even if it is an abnormal cell. Charging an abnormal cell may break the cell.
A numeral 404 shows a positive pole of the secondary cell 401 and a numeral 411 shows a negative pole of the secondary cell 401.
Numerals 404 and 405 show an external terminal, respectively
A numeral 406 shows a signal line and a numeral 409 shows a load.
To solve the above-described problem, an object of the present invention is to provide a high reliability and safe charge and discharge control circuit which does not permit charge current to be pass through the cell to charge when voltage of the cell is less than the threshold voltage of the MOS transistor and the charger is connected.